Wavelength modulation spectroscopy, such as described in U.S. Pat. No. 5,637,872 and the references cited therein, requires a reference gas absorption cell. The cell has the dual purpose of providing an absorption wavelength reference and an absorption magnitude reference. Typically the reference cell is periodically switched into the beam path. The wavelength spectrometer measures the absorption as the laser wavelength is scanned across the gas absorption line, and the laser center wavelength is adjusted so that absorption occurs at a convenient part of the scan. The peak of the gas absorption might for example be centered on the scan or it may be at the extremities of the scan if an adjacent atmospheric absorption interferes with the gas absorption. The tunable laser is usually a semiconductor diode laser, which may be tuned either using bias current or operating temperature. Because electronic sensitivity changes, with for example electronic drift and offset, the reference and offset gas cell may be used to calibrate the spectrometer each time the cell is switched into the beam path. In this step the peak absorption of the reference cell is recorded and stored. External unknown absorption is then compared with the recorded reference recorded absorption and the external absorption is obtained ratiometrically rather than using an electronic signal level.
In the art, gas absorption cells typically consist of elongated tubes sealed at the ends with thin windows. Etalon interference is well known in wavelength absorption spectroscopy so that reference cell windows must be carefully designed. In one method, described in the aforementioned patent, the windows have a wedge shape with a defined angle so that etalon interference occurs at a frequency outside the sensitivity of the spectrometer. In another method using HF gas, glass windows cannot be used since this gas reacts with the windows. Prior art gas absorption cells used to contain, for example, HF and HCl have been fabricated from gold plated Inconel.TM. metal with thin sapphire windows bonded onto the tube using fluoridated O-ring seals. Even after careful processing to remove grease and trapped water these cells must be periodically refilled and since the gas concentration in the cell falls with time, usually in an unpredictable manner, these cells cannot be used for ratiometric calibration. Typically, as shown in FIG. 3, very thin glass windows 50, 52 are bonded onto a reference cell tube 48 at an angle to the tube axis 58. A filling port 54 and evacuation port 56 are also provided for filling and emptying the tube 48 with gas 60. An angle of 10.degree. and a window thickness of 0.5 mm are typical. Prior art gas reference cells are difficult to fabricate for the containment of reactive gases such as hydrogen fluoride. This gas is highly toxic and corrosive and is used in aluminum smelters, refineries and other industrial processes.
HF gas in a standard gas absorption cell is very rapidly consumed by the cell. The gas reacts with water vapor released by the cell walls and with the material of the cell walls. In the case of a gas reference cell once a laser interferometer is installed into an industrial facility, servicing requires a service visit by a trained technician who will refill the cell and recalibrate the instrument with an external cell to meet regularity requirements. Such an external calibration cell for reactive gases requires a "flow through" system where gases containing a small concentration of, for example, HF are flushed through the cell until equilibrium is reached. Releasing toxic gas such as HF into an enclosure such as a building is of course not usually possible which makes in situ calibration difficult.